You put an @ symbol in front of the argument when you call it. For example calling it might look like
[status, callbackData] = DAQmxRegisterEveryNSamplesEvent(task, everyNsamplesEventType,...
nSamples, options, DAQmxEveryNSamplesEventCallbackPtr, @MyCallbackFunction, callbackData)
where you've assigned values for the other arguments. Then you'd define that MyCallbackFunction() function somewhere, like
function results = MyCallbackFunction()
% code...
% The function needs to know what will be given to it when DAQmxRegisterEveryNSamplesEvent() gives stuff to it
% and what kind of data DAQmxRegisterEveryNSamplesEvent() expects to get back after MyCallbackFunction has been executed.
